In radiological imaging with a computer tomography (CT) device for example, materials with high density, e.g. tooth fillings or artificial hips, lead to a complete absorption of the x-radiation on which the imaging is based. Such interfering elements in the area of a patient to be examined radiographically thus cause the loss of evaluable projection data. This means that the reconstruction of an image data set from such raw data for anatomical areas and which lie in the direction of radiological examination in front of and behind the interfering elements with high-density becomes imprecise and no longer meaningful. The effects resulting from an inadequately executable reconstruction in a 3D image data record are referred to as metal artifacts. The effects are not restricted to the region of the material with the high density but also affect other radiographic layers in which the interfering element lies.
A radiological imaging restricted in this way has a particular effect on the accuracy of therapy planning for a radiation therapy, especially for a particle therapy. In therapy planning dose and/or extent are calculated precisely on the basis of the density distribution measured during CT imaging. The decisive factor in the extent calculation and thereby for the accuracy of the dose distribution is the availability of a very precise density distribution of the tissue, measured in HU (Houndsfield Units) as a measure of the attenuation of x-ray radiation. This is because the local attenuation of the radiation in the patient is calculated by means of the HUs obtained from a radiology planning CT recording. An improved, i.e. reduced-artifact imaging thus has a positive effect on accuracy in therapy planning.
Usually layers and angular areas in which complete absorption of the radiation has occurred are determined for the imaging and therapy planning in the imaging data set before the reconstruction and are interpolated with surrounding projection data/raw data. In this case it is ensured that the projection data is completed as realistically as possible. Another procedure which is currently being applied in radiation therapy is to manually set the HU values in the CT imaging area which are affected by metal artifacts to a value which is to a certain degree sensible. Despite this, radiation directions of therapy radiation/therapy particle radiation in which not-insignificant metal artifacts have occurred are often excluded for radiological examination. Alternatively these types of interfering elements can for example be mechanically removed or, such as with the tooth fillings for example, be replaced by non-interfering materials such as plastic for example.